CN115232914B - Method for improving modification effect of ship plate steel magnesium - Google Patents

Abstract

The invention discloses a method for improving the magnesium modification effect of ship plate steel. In the LF refining process, the components of the refining slag are adjusted to the low melting point region, the slag basicity is 4-7, and CaO/Al ₂ O ₃ is 1.5-2.0 between, the Mannesmann index MI is 0.25-0.35, and the key influencing factors to control Mg modification before magnesium treatment in the final stage of LF refining are w _S ≤ 0.008%, w _Al + w _S ≤ 0.055%, w _Ca ≤ 4ppm, activity Oxygen meets a _O ≤ 5ppm, and the magnesium content w _Mg added according to the Al content in the steel meets 0.0028w _Al +8.8×10 ^‑6 to 0.006w _Al +4.1×10 ^‑5 , and the line is accurately fed in sections Methods The magnesium-containing cored wire was fed into the molten steel, and the Al ₂ O ₃ inclusions in the ship plate steel were modified into fine and dispersed MgO‑Al ₂ O ₃ inclusions. The mass percentage of MgO in MgO‑Al ₂ O ₃ was 22～100%, the oxide inclusion types are mainly MgO·Al ₂ O ₃ and MgO, after modification, MgO‑Al ₂ O ₃ can be wrapped by MnS or (Mn,Mg)S to form composite inclusions, so that sulfide inclusions can also get refined. Adopting the method of the invention can ensure that the line feeding process is stable, the inner wall of the nozzle has no obvious nodules, and can smoothly realize multi-furnace continuous pouring. The product has good surface quality, low defect rate and excellent performance.

Description

A method for improving the magnesium modification effect of ship plate steel

technical field

The invention relates to the technical field of ship plate steel production, in particular to a method for improving the magnesium modification effect of ship plate steel, which is applied in the technical field of iron and steel metallurgy.

Background technique

Ship plate steel, that is, steel for hull structure, is mainly used to manufacture hulls and decks of oceangoing, coastal and inland waterway ships. The sailing environment of the ship is harsh, and the shell of the hull not only has to withstand the chemical, electrochemical, and marine biological and microbial corrosion of seawater during service, but also bears the impact of large wind and waves and alternating loads, and in the process of manufacturing and assembling the hull, The material itself will generate a large internal stress. Therefore, it is required that the steel for ship plates should have properties such as small density-to-mass ratio, high strength, good toughness, corrosion resistance, and good weldability. The impurity elements and inclusions in ship plate steel are important factors affecting the performance of steel in all aspects, so in the production process, the quality of molten steel should be improved, including purity, composition, gas content, etc., and reasonable inclusion control methods should be adopted To meet the quality requirements of ship steel.

Al deoxidation is usually used in the production process of ship plate steel, resulting in a large amount of Al ₂ O ₃ inclusions. Al ₂ O ₃ is easy to gather and grow to form clusters and inclusions, or it is easy to adhere to the inner wall of the nozzle during the casting process, causing nodules at the nozzle, and in severe cases, the nozzle is blocked and the pouring is interrupted, affecting the smooth production and reducing production efficiency. The clustered Al ₂ O ₃ adhered and gathered at the nozzle will also enter the crystallizer under the action of molten steel erosion, and be captured by the solidified slab shell during the continuous casting process, becoming large-sized inclusions in the slab. During product quality testing, problems often arise due to excessive Al ₂ O ₃ inclusions leading to flaw detection failures. Al ₂ O ₃ inclusions have a high melting point, high hardness, and poor deformation ability. They are quite different from the properties of the steel matrix. During the rolling process, they are not easy to deform with the matrix, thus forming voids around the inclusions and becoming the source of cracks. Al ₂ O ₃ inclusions in steel, especially large-sized Al ₂ O ₃ inclusions, are important factors that lead to low pass rate of flaw detection, poor performance stability, and low yield of finished products, which are harmful to all aspects of steel performance and reduce product quality. .

Magnesium treatment technology is one of the effective measures to control Al ₂ O ₃ inclusions, which can transform Al ₂ O ₃ inclusions into fine and dispersed inclusions, and eliminate the harm of large-sized Al ₂ O ₃ . Fine and dispersed inclusions can be used as heterogeneous nucleation points during the solidification process of molten steel to promote grain refinement. In addition, fine inclusions can pin austenite grain boundaries, inhibit grain growth, and can Induces the formation of intragranular acicular ferrite to further refine the structure. During high heat input welding, during the welding cooling process, the structure of magnesium-treated steel in the welding heat-affected zone is refined, thereby improving the strength and toughness of the welded joint and heat-affected zone, thereby improving welding performance. However, in the process of Mg treatment, there will be problems such as violent reaction, low element yield, and unstable product quality. In addition, if the amount of Mg added is unreasonable or the process is not properly controlled, there will be a risk of nodulation at the nozzle, resulting in magnesium Applications of processing techniques are limited.

The Chinese patent with the publication number of patent document CN106399633B discloses "a kind of magnesium treatment process for ship plate steel molten steel". After the alloy fine-tuning in the LF process or after the vacuum treatment in the RH process, Mg-Al-Fe alloy cored wire is added, and the core The mass percentage of the composition in the wire is Mg: 5-15%, Al: 40-65%, and the balance is Fe and unavoidable impurities. Under a certain cored wire structure, the wire feeding speed is 2.5-4.0m/s Feed at a radius of 1/3~1/2 from the center of the ladle, and then perform a certain soft blowing to achieve the purpose of controlling the cleanliness of the ship plate steel and the composition, quantity, particle size and distribution of inclusions in the steel, thereby improving the ship plate Mechanical properties of steel and its stability. This technology is a conventional line-feeding process, and the target control of the line-feeding is not precise enough. The influence of the slag system on the effect of magnesium addition is not considered, and there are still risks or problems such as violent reaction, unstable product quality, and nodules at the water outlet.

The Chinese patent with the publication number CN 109536840A discloses "a kind of continuous casting high-quality mold steel with micro-magnesium treatment and its preparation method". The composition of magnesium-containing mold steel is designed and the corresponding production process is proposed. Magnesium alloy wire is added after gas treatment, the composition of the alloy wire is Al: 30%-50%, Mg: 10%-25%, the balance is Fe, and the wire core weight is 180-220g/m. Add it into molten steel at a certain feeding speed and feeding amount to play the role of deoxidation purification and modification of inclusions, thereby improving the purity and toughness of steel. This technology is a conventional line-feeding process, and the target control of the line-feeding is not precise enough. The influence of the slag system on the effect of magnesium addition is not considered, and there are still risks or problems such as violent reaction, unstable product quality, and nodules at the water outlet.

The Chinese patent with the publication number of patent literature CN 110117694B discloses the "Magnesium Addition Process for Free Cutting Steel Containing Magnesium". for:

1) Control the production conditions before feeding the line, control the slag layer, temperature and oxygen activity of molten steel;

2) Feeding of magnesium-containing cored wire;

3) Processing after the wire feeding is finished.

The process aims to improve the form of manganese sulfide inclusions in the steel, reduce the deformation during rolling, and further improve the cutting performance of the steel. It is suitable for the production of free-cutting stainless steel, and the sulfur content in the steel is 0.15-0.35%. This technology focuses on the magnesium addition process and refines the condition control during the magnesium addition process. However, the feed line amount of the inclusion control target is not precise enough, the interrelationship control of elements that are likely to cause nozzle nodules is not clear enough, and the slag system has no effect on magnesium treatment. The importance of the product has not attracted enough attention, and the stability of product quality cannot be guaranteed.

The Chinese patent with the publication number CN 113278763A discloses "a product and method for treating molten steel with magnesium or magnesium-calcium". Combined with diluent to form a miscible alloy melt, thereby stabilizing, diluting and dispersing metallic magnesium, it can effectively inhibit the rapid evaporation of magnesium during the magnesium modification and denaturation treatment of inclusions in steel, and reduce the gasification and oxidation loss of magnesium. Improve and stabilize the yield of magnesium in steel, and ensure the purity of molten steel after magnesium treatment. This technology uses the reaction of magnesium oxide and composite reducing agent to generate magnesium source, combined with a stable diluent to reduce the severity of the reaction and reduce the gasification and oxidation loss of magnesium, but the introduced magnesium oxide causes oxygenation of molten steel , which is detrimental to the cleanliness of the molten steel, and the control of the feeding amount is not precise enough, the target inclusions are not clear enough, and the influence of the slag system on the effect of magnesium addition is not considered, and there are still risks or problems such as unstable product quality and easy nozzle nodules.

The Chinese patent with the publication number CN 112195305A discloses "a method for refining the grain size of sulfur-containing non-quenched and tempered steel", which ensures that the active oxygen in the steel is 5-15ppm, feed a certain amount of magnesium-silicon cored wire to make the magnesium content in the steel reach 0.001%-0.003%, use Mg element to modify MnS inclusions, and form fine and dispersed manganese sulfide and magnesium-aluminum oxide compound Inclusions, as the nucleation core of ferrite, induce the formation of equiaxed ferrite and refine the grains. This technology is not precise enough to control the feeding amount, the target inclusions are not clear enough, and the influence of the slag system on the effect of magnesium addition is not considered. There are still risks or problems such as unstable product quality and nodules at the water outlet.

Magnesium treatment technology plays a significant role in improving the cleanliness of molten steel, improving Al ₂ O ₃ and MnS inclusions in steel, and refining grains. The quality of the product is not stable, and the industrial production is still greatly restricted. Two of the main problems are nodulation at the nozzle and the stability of the effect of adding magnesium. These two aspects are not clearly involved in the prior art, and this has become a technical problem that needs to be solved urgently. .

Contents of the invention

In order to solve the problems in the prior art, the invention provides a method for improving the magnesium modification effect of ship plate steel. Control the Al, S, and Ca elements in the steel before Mg treatment to prevent the occurrence of nozzle nodules, determine the optimal Mg addition amount by controlling the oxygen activity of molten steel and put forward the relationship between Mg and Al content, and clarify inclusions Control the type and mass ratio of components to ensure good Mg treatment effect. In addition, combined with the optimization of refining slag system, excessive large-size inclusions can be removed by adsorption, the cleanliness of molten steel can be improved, and the Mg treatment process and product quality can be stabilized.

In order to achieve the above object, the present invention adopts following technical scheme to realize:

A method to improve the effect of magnesium modification of ship plate steel. In the production process of ship plate steel, the LF refining slag system with low melting point, high Al ₂ O ₃ absorption rate and low Mg consumption rate is adjusted and controlled in the LF refining process. Before the final magnesium treatment, control the content of Al, S, Ca, O and the degree of superheat, which are the key factors affecting magnesium modification, and determine the appropriate quantitative addition range of Mg according to the Al content in the steel. The core wire is fed into the molten steel, and the Al ₂ O ₃ inclusions in the ship plate steel are modified by magnesium treatment into fine and dispersed MgO-Al ₂ O ₃ , and the modified MgO-Al ₂ O ₃ is mostly replaced by MnS or (Mn ,Mg)S wrapping, forming composite inclusions; the size of MgO-Al ₂ O ₃ is mainly 0.5-3μm, and the size of the overall composite inclusions is mainly 0.5-6μm; in addition, the average size of all inclusions is 2-4μm, and the size is less than 2μm The proportion of inclusions is more than 40%, and the proportion of inclusions larger than 10μm is less than 0.5%.

Preferably, the production process of ship plate steel is: converter → LF refining → Mg treatment → continuous casting; the composition of the finished ship plate steel is: C: 0.05-0.09%, Si: 0.1-0.3%, Mn: 1.2- 1.5%, P≤0.015%, S≤0.01%, Nb: 0.02～0.04%, Ti: 0.01～0.02%, Al: 0.02～0.05%, Mg: 0.0005～0.005%, the balance is iron and unavoidable impurities .

Preferably, the method of controlling the LF refining slag system with low melting point, high _Al2O3 absorption rate and low Mg consumption rate that is beneficial _to magnesium treatment is as follows:

In the process of converter tapping, add lime and top slag modifier, and in the process of LF refining, add slagging material so that the composition of refining slag satisfies the following ratio by mass percentage:

CaO: 42-55%, SiO ₂ : 5-12%, MgO: 6-10%, Al ₂ O ₃ : 20-30%, MnO+FeO≤1%, CaF ₂ ≤6%;

And the basicity of the slag is 4-7, the CaO/Al ₂ O ₃ is between 1.5-2.0, and the Mannesmann index MI is 0.25-0.35. Slag has a lower melting point, better desulfurization ability and good fluidity, which is beneficial to the adsorption and removal of inclusions and can avoid the secondary oxidation of molten steel caused by the oxidative properties of slag.

Preferably, the Al, S, Ca, O content and degree of superheat, which are key factors affecting magnesium modification before magnesium treatment, meet the following requirements:

At the end of LF refining, before feeding the magnesium-containing cored wire, ensure that the mass percentages of Al, S, and Ca elements in molten steel meet w _S ≤ 0.008%, w _Al + w _S ≤ 0.055%, w _Ca ≤ 4ppm, and the active oxygen meets a _O ≤5ppm, the degree of superheat is 40～65℃.

Preferably, MgO-Al ₂ O ₃ is a pure MgO·Al ₂ O ₃ phase or a solid solution of Al ₂ O ₃ or MgO in MgO·Al ₂ O ₃ , or a mixed phase of MgO·Al ₂ O ₃ and MgO; MgO - The mass percentage of MgO in Al ₂ O ₃ is 22-100%; the pure MgO·Al ₂ O ₃ phase is magnesium aluminum spinel with a stoichiometric ratio of MgO to Al ₂ O ₃ of 1:1.

Preferably, the appropriate Mg quantitative addition range is determined according to the Al content in the steel as follows:

The magnesium content w _Mg obtained by feeding the magnesium-containing cored wire into the molten steel is between 0.0028w _Al +8.8×10 ^-6 and 0.006w _Al +4.1×10 ^-5 , and the inclusions formed in the steel MgO-Al ₂ O ₃ The mass percentage of MgO is 22-100%, and the oxide inclusion types are mainly MgO·Al ₂ O ₃ and MgO.

Further preferably, the magnesium content w _Mg ranges from 0.0028w _Al +8.8×10 ^-6 to 0.0029w _Al +1.5×10 ^-5 , and the mass percentage of MgO in the inclusions MgO-Al ₂ O ₃ formed in the steel is 22-35%, the oxide inclusion type is mainly MgO·Al ₂ O ₃ .

Preferably, the steps of feeding the magnesium-containing cored wire into the molten steel by the segmented precise wire feeding method are as follows:

When feeding the wire, if the feeding length of the core-spun wire is less than 200m, feed it in two times, and the two feeding amounts account for 0.382 and 0.618 of the total feeding wire length respectively according to the golden ratio;

If the feeding length of the cored wire is greater than 400m, it can be fed in 3 times, and the feeding amount of the three times accounts for 1/4, 1/3, 5/12 of the total feeding wire length;

If the feeding amount of the cored wire is 200-400m, it can be fed in 2 or 3 times; when feeding the magnesium-containing cored wire, the feeding speed is 50-200m/min, and the time interval between two feedings is 10 ~20s, to ensure a stable wire feeding process; after the wire feeding is finished, after soft blowing for 5~15 minutes, the molten steel is continuously cast.

Further preferably, the method for determining the wire feeding speed is as follows: according to the ladle molten pool depth h meters, the cored wire diameter D mm, and the cored wire iron sheet thickness δ mm, determine the wire feeding speed as v=(h-0.15)·δ ^-1 ·(1-δ/D) ^-1 ·D ^-0.5 .

Principle of the present invention:

In the refining process of ship plate steel, Al ₂ O ₃ produced by Al deoxidation has a relatively large wetting angle with molten steel, and is easy to aggregate and grow to form large-sized inclusions. In addition, in the continuous casting process, Al ₂ O ₃ is easy to adhere to the inner wall of the nozzle and form nodules, resulting in nozzle blockage. The nodules enter the crystallizer under the action of molten steel scouring, forming larger-sized inclusions, which are captured by the solidified billet shell and become defects in the steel. Al ₂ O ₃ inclusions have a high melting point and high hardness, which have a very adverse effect on the quality and performance of steel. How to control Al ₂ O ₃ inclusions is the key to reducing its harm.

The modification process of Mg on Al ₂ O ₃ inclusions is usually a process of transformation from the outer surface of the inclusions to the inner surface. The larger the size of the inclusions, the greater the diffusion resistance of Mg to the interior of Al ₂ O ₃ , and the more difficult the modification is. In order to fully modify the Al ₂ O ₃ inclusions in the steel, before adding Mg, the Al ₂ O ₃ inclusions in the steel should be made as small and medium-sized inclusions as possible, and the large-sized inclusions should be removed. According to the Stokes formula, the large-size inclusions float faster and are easier to float to the slag-steel interface. By adjusting the slag system, the adsorption and removal of large-size inclusions can be promoted. During the LF refining process, appropriate slagging materials are added to make the refining slag components satisfy the following mass percentages: CaO: 42-55%, SiO ₂ : 5-12%, MgO: 6-10%, Al ₂ O ₃ : 20- 30%, MnO+FeO≤1%, _CaF2≤6 %. This composition range can make the slag have a lower melting point and good fluidity, _and the liquid _slag layer can evenly cover the surface of the molten steel. During the core wire process, it can reduce the volatilization path of Mg vapor, reduce the burning loss of Mg, and increase the yield of Mg.

The slag contains 6-10% MgO. On the one hand, the MgO in the slag can be saturated as much as possible to reduce the erosion of the furnace lining magnesia carbon brick. The reaction between them reduces the loss of dissolved Mg in steel and increases the yield of Mg. Too high MgO content will increase the melting point of slag, while too low MgO content will easily corrode the lining refractory and increase the reaction loss of Mg between slag and steel. On the premise of ensuring the fluidity of slag, the slag alkalinity is controlled at 4-7, which can play a good desulfurization effect. Too high slag basicity will increase the melting point of slag and deteriorate the fluidity. Too low slag alkalinity The degree is unfavorable for desulfurization. In addition, if the CaO/Al ₂ O ₃ in the slag is controlled between 1.5 and 2.0, and the Mannesmann index MI is 0.25 to 0.35, the slag can have a suitable viscosity and a strong Al ₂ O ₃ adsorption capacity, which can effectively Removing large-sized Al ₂ O ₃ , improving the cleanliness, is conducive to the subsequent sufficient modification of Al ₂ O ₃ inclusions by Mg.

When Al ₂ O ₃ is modified into MgO-Al ₂ O ₃ , if the MgO content in MgO-Al ₂ O ₃ is 28.2%, it is a pure magnesium aluminum tip with a stoichiometric ratio of MgO to Al ₂ O ₃ of 1:1 Spar MgO·Al ₂ O ₃ phase. When the MgO content in MgO-Al ₂ O ₃ is 22-35%, the magnesium aluminum spinel MgO·Al ₂ O ₃ can solid dissolve a certain amount of MgO or Al ₂ O ₃ . When the MgO content in MgO-Al ₂ O ₃ is lower than 22%, non-solid solution Al ₂ O ₃ will be produced. When the MgO _{content in} MgO- _Al2O3 is higher than 35%, there will be MgO· _Al2O3 and MgO.

The critical nucleation radius of MgO·Al ₂ O ₃ or MgO is smaller than that of Al ₂ O ₃ , the nucleation rate is high, and it is distributed in a finer and dispersed state in molten steel. The interface energy of MgO·Al ₂ O ₃ or MgO and molten steel is smaller than that of Al ₂ O ₃ and molten steel, the wetting angle is also smaller, and the attraction force between particles is also smaller than that of Al ₂ O ₃ , so it is not easy to aggregate and grow , to avoid the formation of cluster-like inclusions, so the final product is also finer and dispersed, reducing the quality defects caused by large particle inclusions, while small-size inclusions usually have no adverse effects on the properties of steel. In addition, MgO·Al ₂ O ₃ or MgO is not easy to accumulate in the nozzle, which reduces the risk of nozzle blockage and ensures continuous pouring. During the solidification process of molten steel, due to the mismatch degree of 1.2% between MgO·Al ₂ O ₃ and δ-Fe or the mismatch degree of 3.8% between MgO and δ-Fe, both Al ₂ O ₃ and δ-Fe The mismatch between 16.1% is smaller, MgO Al ₂ O ₃ or MgO is more likely to induce the formation of intragranular acicular ferrite, thereby refining the grain, improving the structure, improving low temperature impact toughness, welding performance, etc. . When the MgO content in MgO-Al ₂ O ₃ is less than 22%, due to the existence of Al ₂ O ₃ , Al ₂ O ₃ may sinter with MgO·Al ₂ O ₃ , which will increase the size of inclusions and stick at the nozzle. Attached sintering will have adverse effects on the performance and quality of steel. Therefore, the MgO content in MgO-Al ₂ O ₃ should be 22-100%.

In an ideal state, without the interference of other elements, the Mg element can modify Al ₂ O ₃ into fine and dispersed MgO-Al ₂ O ₃ , but in the actual production process, there will inevitably be some amount of Ca and S. Part of Ca exists in the form of CaO, which combines with Al ₂ O ₃ to form a high melting point CaO-Al ₂ O ₃ , or combines with MgO and Al ₂ O ₃ to form a complex stoichiometric ternary CaO-MgO-Al ₂ O ₃ inclusion , whether CaO-Al ₂ O ₃ or ternary inclusion CaO-MgO-Al ₂ O ₃ loses the characteristics of MgO·Al ₂ O ₃ or MgO surface tension, wetting angle, critical nucleation radius, etc., making CaO-Al ₂ O ₃ or CaO-MgO-Al ₂ O ₃ are easy to aggregate and grow together. On the other hand, CaS formed by the combination of Ca and S is easy to adhere to and sinter with MgO-Al ₂ O ₃ on the inner wall of the nozzle, resulting in nodulation of the nozzle. Therefore, at the end of LF refining, during Mg treatment, the contents of S and Ca in steel should be strictly limited to ensure that the mass percentages of S and Ca elements in molten steel satisfy w _S ≤ 0.008%, w _Ca ≤ 4ppm. In addition, when the content of S and Al is too much at the same time, it is easier to form inclusions such as high melting point CaO-Al ₂ O ₃ , CaS, CaO-MgO-Al ₂ O ₃ , so the total amount of S and Al needs to satisfy w _Al + w _S ≤0.055%. Mg has strong chemical activity and has a strong binding force with oxygen. When the active oxygen in the steel is high, the added Mg will react violently, causing a large amount of Mg to be used for deoxidation, which cannot achieve the effect of inclusion modification. Therefore, the active oxygen in the molten steel needs to meet a _O ≤ 5ppm.

When feeding magnesium-containing cored wire, determine the appropriate quantitative addition range of Mg according to the Al content in the steel, and the magnesium content w _Mg obtained in molten steel after feeding the wire should satisfy 0.0028w _Al +8.8×10 ^-6 to 0.006w _Al +4.1×10 Between ^-5 , the mass percentage of MgO in the inclusions MgO-Al ₂ O ₃ formed in the steel is 22-100%, and the oxide inclusion types are mainly MgO·Al ₂ O ₃ and MgO. The preferred range of magnesium content w _Mg is between 0.0028w _Al +8.8×10 ^-6 to 0.0029w _Al +1.5×10 ^-5 , and the mass percentage of MgO in the inclusions MgO-Al ₂ O ₃ formed in the steel is 22～ 35%, the type of oxide inclusions is mainly MgO·Al ₂ O ₃ . If the w _Mg content is lower than 0.0028w _Al +8.8×10 ^-6 , there will be Al ₂ O ₃ inclusions, resulting in under-modification; if the w _Mg content is higher than 0.006w _Al +4.1×10 ^-5 , there will be only MgO Formation, at this time, Mg is added excessively, Mg is too late to dissolve and diffuse, and is volatile, resulting in a significant decrease in the yield of Mg, and the addition of too much will inevitably increase the intensity of the wire feeding reaction. No matter under-modified or over-modified, it is unfavorable to the treatment effect of Mg.

In the wire feeding process, if the feeding length of the cored wire is less than 200m, it can be fed in two times, and the length of the two feeding wires is controlled to 0.382 and 0.618 according to the golden section ratio, and the time interval between the two feeding times is 10~ 20s. In the first feeding, the active oxygen in the steel is relatively high, and the cored wire fed mainly plays a role in deoxidation. Therefore, a small amount of feeding can avoid excessive reaction, and at the same time, ensure a certain local Mg concentration, which not only plays a preliminary role. Modification effect, and at the same time make it gradually diffuse to other parts of molten steel during the interval before the next wire feeding. In the second feeding, the feeding amount is increased, and the inclusions are fully modified during the smooth addition of Mg. If the feeding length of the cored wire is greater than 400m, it can be fed in 3 times, and the feeding amount of the 3 times accounts for 1/4, 1/3, 5/12 of the total feeding line length. The time interval between every two feedings is 10-20s, the first feeding amount is less, and the third feeding amount is more. In the first feeding, the active oxygen in the steel is relatively high, and the fed cored wire is mainly used for deoxidation, so a small amount of feeding is used to avoid excessive reaction. In the second feeding, the feeding amount should be increased appropriately. At the same time of further deoxidation, the effect of inclusion modification is gradually achieved, and at the same time, a certain local Mg concentration is ensured, so that it gradually diffuses to other parts of the molten steel during the interval before the next feeding line, and the third feeding amount is further increased. , In the process of adding Mg smoothly, the inclusions are fully modified. If the feeding amount of the cored wire is 200-400m, it can be fed in 2 or 3 times.

In order to ensure the smooth progress of the wire feeding process and ensure the yield of Mg, it is necessary to set the wire feeding parameters reasonably. Usually, the speed of feeding magnesium-containing cored wire is 50-200m/min. If the wire feeding speed is too slow, the cored wire will be melted in the upper part of the molten pool, and the alloy elements will be volatilized and burned before they are dissolved, resulting in violent reaction and low yield. If the wire feeding speed is too fast, the cored wire will scratch the refractory material at the bottom of the molten pool. The more accurate wire feeding speed is related to the depth of ladle molten pool h (unit m) and the melting time t (unit s) of the iron sheet of the cored wire. Usually the best melting position of the end of the cored wire is located at 100-200 mm from the bottom of the ladle , take 150mm here. The best feeding speed v (unit m/s) is shown in formula 1. The melting time t of the cored wire iron sheet is related to the cored wire diameter D (unit mm) and the cored wire iron sheet thickness δ (unit mm), and the corresponding relationship is shown in formula 2. From this, the optimal wire feeding speed can be determined as shown in Equation 3 to ensure the smooth progress of the wire feeding process.

v＝(h-0.15)/t (1)

t=δ·(1-δ/D)·D ^0.5 (2)

v=(h-0.15)·δ ^-1 ·(1-δ/D) ^-1 ·D ^-0.5 (3)

In the subsequent continuous casting process and cooling process after solidification, MnS will be precipitated from the steel. Due to the good plasticity of MnS, during the rolling process, MnS is easy to deform with the matrix to form long strip inclusions, which affects the impact toughness of ship plate steel and reduces the HIC resistance of steel. When the steel contains oxide inclusions, MnS will precipitate from the steel and use the oxide inclusions in the steel as heterogeneous nucleation sites to form composite inclusions. Oxide inclusions, as the hard core of MnS, can effectively suppress the deformation of MnS inclusions during rolling and keep them in a spindle shape with a small aspect ratio. When the Al ₂ O ₃ in the steel is regulated to be MgO-Al ₂ O ₃ , composite inclusions of MgO-Al ₂ O ₃ and MnS will be formed. Since MgO-Al ₂ O ₃ is more finely dispersed than Al ₂ O ₃ , it can provide more MnS heterogeneous nucleation sites, which increases the proportion of composite inclusions and reduces the size of the overall inclusions in the steel. reduce. Composite inclusions can improve the deformation resistance of more MnS inclusions and reduce the aspect ratio of the overall inclusions after rolling. In addition, Mg can be dissolved into MnS to form (Mn,Mg)S solid solution, which increases the hardness of MnS and is not easy to deform during rolling. It can also reduce the damage of MnS inclusions to steel properties and improve product quality.

Compared with the prior art, the present invention has the following obvious outstanding substantive features and significant advantages:

1. By adopting the method of the present invention, the wire feeding process can be ensured to be stable without violent reaction, the yield of Mg element is high, the stopper rod curve is stable during the continuous casting process, there is no obvious nodulation phenomenon on the inner wall of the nozzle, and multi-furnace continuous casting can be smoothly realized;

2. The method of the present invention can accurately control the formation type of inclusions. Most of the inclusions exist as composite inclusions of fine and dispersed MgO-Al ₂ O ₃ and MnS or (Mn,Mg)S, and the size of MgO-Al ₂ O ₃ is mostly 0.5-3μm, the overall size of composite inclusions is mostly 0.5-6μm; in addition, the average size of all inclusions is 2-4μm, the inclusions with a size below 2μm account for more than 40%, and the proportion of inclusions larger than 10μm is less than 0.5%;

3. The ship plate steel produced by the technology of the present invention has good surface quality, low defect rate and excellent performance; the rolled material prepared by the method of the present invention is not higher than 1.0 according to the B class rating in the national standard GB/T 10561. Grade; determined according to ASTM E45 method A, the qualified rate of inclusions grade 1.0 is over 95%.

Description of drawings

Fig. 1 is a photo of CaO-Al ₂ O ₃ inclusions in ship plate steel under the traditional Ca treatment process in Comparative Example 1.

Fig. 2 is a photograph of CaO-Al ₂ O ₃ -MgO+CaS inclusions in ship plate steel under the Mg treatment process of Comparative Example 2.

Fig. 3 is a photo of MgO-Al ₂ O ₃ inclusions in ship plate steel under the Mg treatment process of Example 1 of the present invention.

Detailed ways

The above scheme will be further described below in conjunction with specific examples. The following examples are only exemplary for explaining and illustrating the technical scheme of the present invention, and cannot be interpreted as limiting the technical scheme of the present invention. Comparative example 1 is Traditional Ca treatment process, comparative example two is the Mg treatment process not of the technology of the present invention, and embodiments one to four are the Mg treatment process of the technology of the present invention, and the preferred embodiments of the present invention are described in detail as follows:

The production process of ship plate steel is: converter → LF refining → Mg treatment → continuous casting; the composition of the finished ship plate steel is: C: 0.05-0.09%, Si: 0.1-0.3%, Mn: 1.2-1.5%, P≤0.015%, S≤0.010%, Nb: 0.02-0.04%, Ti: 0.01-0.02%, Al: 0.02-0.05%, Mg: 0.0003-0.0020%, and the balance is iron and unavoidable impurities.

In the converter tapping process, add lime and top slag modifying agent, in the LF refining process, add other slagging materials appropriately, the refining slag components when LF leaves the station are shown in Table 1, and the contents of each component in the present invention The slag basicity is between 4 and 7, the CaO/Al ₂ O ₃ is between 1.5 and 2.0, and the Mannesmann index MI is between 0.25 and 0.35. Slag has a lower melting point, better desulfurization ability and good fluidity.

At the end of LF refining, before feeding the magnesium-containing cored wire, the content of S, Al, and Ca elements, the active oxygen value and the degree of superheat are shown in Table 2, and each composition and temperature meet the wire feeding conditions. According to the Al content, feed magnesium-containing cored wire in the range of 0.0028w _Al +8.8×10 ^-6 to 0.0029w _Al +1.5×10 ^-5 , the thickness of the iron sheet of the cored wire is 0.3mm, and the diameter of the cored wire is 9mm. The depth of the pool is about 2.2m. The feeding speed, feeding frequency and feeding amount of the cored wire are shown in Table 3. The time interval between two feedings is 15s. The wire feeding process is stable. Finally, the molten steel was continuously casted, and the Mg content in the final steel was shown in Table 2.

Table 1. LF outbound refining slag composition and control indicators

Table 2. Contents of key elements and degree of superheat before feeding the wire and magnesium content in the final steel

By adopting the technical scheme described in the above-mentioned embodiments of the present invention, the wire feeding process is stable without violent reaction, the stopper rod curve is stable during the continuous casting process, and there is no obvious nodulation phenomenon on the inner wall of the nozzle, 10 furnaces of continuous casting are successfully realized, and the yield of Mg element is relatively high. The conventional magnesium treatment process has been significantly improved, as shown in Table 3. Most of the inclusions exist in the form of fine and dispersed MgO·Al ₂ O ₃ or composite inclusions of MgO· _{Al 2 O 3} and MnS, (Mn,Mg)S. The size of the inclusions is mostly 0.5-6 μm. In addition, the average size of all inclusions is 2-4 μm, and the proportion of inclusions larger than 10 μm is below 0.5%. The ship plate steel produced by the technology of the invention has good surface quality, low defect rate and excellent performance. According to the national standard GB/T10561, the rating of grade B of the rolled material is not higher than grade 1.0 for both coarse and fine grades, and can reach grade 0.5 in most cases. According to ASTM E45 method A, the qualified rate of inclusions 1.0 is over 95%. The specific test results are shown in Table 4.

Table 3. Key production process parameters

Table 4. Detection results of inclusions in steel

The method for improving the magnesium modification effect of ship plate steel in the above-mentioned embodiments is to control the composition of the refining slag to the low melting point region during the LF refining process, the slag basicity is 4-7, and the CaO/Al ₂ O ₃ is between 1.5-2.0. The Mannesmann index MI is 0.25-0.35, and the key influencing factors to control Mg modification before magnesium treatment in the final stage of LF refining are w _S ≤ 0.008%, w _Al + w _S ≤ 0.055%, w _Ca ≤ 4ppm, and the activity of oxygen satisfies a _O ≤ 5ppm, and the magnesium content w _Mg added according to the Al content in the steel is between 0.0028w _Al + 8.8×10 ^-6 and 0.006w _Al + 4.1×10 ^-5 The magnesium cored wire is fed into the molten steel, and the Al ₂ O ₃ inclusions in the ship plate steel are modified into fine and dispersed MgO-Al ₂ O ₃ inclusions, and the mass percentage of MgO in MgO-Al ₂ O ₃ is 22-100 %, oxide inclusions are mainly MgO·Al ₂ O ₃ and MgO, and MgO-Al ₂ O ₃ can be wrapped by MnS or (Mn,Mg)S after modification to form composite inclusions and refine sulfide inclusions . By adopting the method of the above embodiment of the present invention, the line feeding process can be guaranteed to be stable, the inner wall of the nozzle has no obvious nodules, and continuous casting of multiple furnaces can be smoothly realized. The product has good surface quality, low defect rate and excellent performance.

The embodiment of the present invention has been described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned embodiment, and various changes can also be made according to the purpose of the invention of the present invention. The changes, modifications, substitutions, combinations or simplifications should all be equivalent replacement methods, as long as they meet the purpose of the invention and as long as they do not deviate from the technical principle and inventive concept of the invention, they all belong to the protection scope of the invention.

Claims (8)

1. A method for improving the magnesium modification effect of ship plate steel, characterized in that: in the ship plate steel production process, the LF refining of low melting point, high Al ₂ O ₃ absorption rate and low Mg consumption rate is adjusted and controlled to facilitate magnesium treatment Slag system, before magnesium treatment in the final stage of LF refining, control the key factors affecting magnesium modification Al, S, Ca, O content and superheating degree, and determine the appropriate Mg quantitative addition range according to the Al content in the steel, and feed the line accurately in sections The method is to feed the magnesium-containing cored wire into the molten steel, and modify the Al ₂ O ₃ inclusions in the ship plate steel into fine and dispersed MgO-Al ₂ O ₃ through magnesium treatment, and the modified MgO-Al ₂ O ₃ Most of them are wrapped by MnS or (Mn,Mg)S to form composite inclusions; The size of MgO-Al ₂ O ₃ is mainly 0.5-3 μm, and the overall composite inclusion size is mainly 0.5-6 μm; In addition, the average size of all inclusions is 2-4 μm, the inclusions with a size smaller than 2 μm account for more than 40%, and the proportion of inclusions larger than 10 μm is less than 0.5%; The composition of the finished ship plate steel is: C: 0.05-0.09%, Si: 0.1-0.3%, Mn: 1.2-1.5%, P≤0.015%, S≤0.01%, Nb: 0.02-0.04%, Ti: 0.01～0.02%, Al: 0.02～0.05%, Mg: 0.0005～0.005%, the balance is iron and unavoidable impurities; The key influencing factors of controlling magnesium modification before magnesium treatment are Al, S, Ca, O content and degree of superheat to meet the following requirements: At the end of LF refining, before feeding the magnesium-containing cored wire, ensure that the mass percentages of Al, S, and Ca elements in molten steel meet w _S ≤ 0.008%, w _Al + w _S ≤ 0.055%, w _Ca ≤ 4ppm, and the active oxygen meets a _O ≤5ppm, the degree of superheat is 40～65℃. 2. The method for improving the magnesium modification effect of ship plate steel according to claim 1, characterized in that: the production process of ship plate steel is: converter→LF refining→Mg treatment→continuous casting. 3. The method for improving _the magnesium modification effect of ship plate steel according to claim 1, characterized in that: adjusting and controlling the LF refining slag system with low melting point, high _Al2O3 absorption rate and low Mg consumption rate that is beneficial to magnesium treatment The method is: In the process of converter tapping, add lime and top slag modifier, and in the process of LF refining, add slagging material so that the composition of refining slag satisfies the following ratio by mass percentage: CaO: 42-55%, SiO ₂ : 5-12%, MgO: 6-10%, Al ₂ O ₃ : 20-30%, MnO+FeO≤1%, CaF ₂ ≤6%; And the basicity of the slag is 4-7, the CaO/Al ₂ O ₃ is between 1.5-2.0, and the Mannesmann index MI is 0.25-0.35. 4. The method for improving the magnesium modification effect of ship plate steel according to claim 1, characterized in that: MgO-Al ₂ O ₃ is a pure MgO·Al ₂ O ₃ phase, or a solid solution of Al ₂ O ₃ or MgO MgO·Al ₂ O ₃ , or a mixed phase of MgO·Al ₂ O ₃ and MgO; the mass percentage of MgO in MgO-Al ₂ O ₃ is 22-100%; the pure MgO·Al ₂ O ₃ phase is MgO and Al ₂ O ₃ stoichiometric ratio of 1:1 magnesium aluminum spinel. 5. the method for promoting the magnesium modification effect of ship plate steel according to claim 1, is characterized in that: according to the Al content in the steel, determine the suitable quantitative addition range of Mg as follows: The magnesium content w _Mg obtained by feeding the magnesium-containing cored wire into the molten steel is between 0.0028w _Al +8.8×10 ^-6 and 0.006w _Al +4.1×10 ^-5 , and the inclusions formed in the steel MgO-Al ₂ O ₃ The mass percentage of MgO is 22-100%, and the oxide inclusion types are mainly MgO·Al ₂ O ₃ and MgO. 6. The method for improving the magnesium modification effect of ship plate steel according to claim 5, characterized in that the magnesium content w _Mg ranges from 0.0028w _Al + 8.8×10 ^-6 to 0.0029w _Al + 1.5×10 ^-5 During the period, the mass percentage of MgO in the inclusions MgO-Al ₂ O ₃ formed in the steel is 22-35%, and the oxide inclusion type is mainly MgO·Al ₂ O ₃ . 7. The method for improving the magnesium modification effect of ship plate steel according to claim 1, characterized in that: the step of feeding the magnesium-containing cored wire into the molten steel by the segmented precise wire feeding method is as follows: When feeding the wire, if the feeding length of the core-spun wire is less than 200m, feed it in two times, and the two feeding amounts account for 0.382 and 0.618 of the total feeding wire length respectively according to the golden ratio; If the feeding length of the cored wire is greater than 400m, it can be fed in 3 times, and the feeding amount of the three times accounts for 1/4, 1/3, 5/12 of the total feeding wire length; If the feeding amount of the cored wire is 200-400m, it can be fed in 2 or 3 times; when feeding the magnesium-containing cored wire, the feeding speed is 50-200m/min, and the time interval between two feedings is 10 ~20s, to ensure a stable wire feeding process; after the wire feeding is finished, after soft blowing for 5~15 minutes, the molten steel is continuously cast. 8. The method for improving the magnesium modification effect of ship plate steel according to claim 7, characterized in that: the wire feeding speed is determined as follows: according to the ladle molten pool depth h meters, the cored wire diameter D millimeters, the cored wire iron sheet The thickness is δmm, and the feeding speed is determined as v=(h-0.15)·δ ^-1 ·(1-δ/D) ^-1 ·D ^-0.5 .

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